This invention relates generally to induction heat treatment, and more particularly, to a device and method for hardening workpieces such as threaded fasteners using a circular conveyor through an induction heating apparatus.
Induction surface hardening of workpieces, such as but not limited to tapping tools and thread-forming screws or bolts, known as self-tapping fasteners, is intended to harden each of the workpieces in a preselected region. The preselected region may comprise a portion of each workpiece or the entire workpiece depending on the type of workpiece heat-treated. Generally, however, it includes the first several threads, as will be more fully described hereinafter.
For thread-forming screws or bolts, it is often desirable to heat-treat only the first 2-4 threads of each workpiece. Generally, this portion comprises a tapered section having several of the thread roots of the workpiece. These thread roots are hardened to perform cutting action or thread-forming action upon entering a substrate to create a threaded region within the substrate. The remainder of the workpiece is then threaded through the newly threaded region of the substrate, thereby providing anchoring support for the substrate and/or objects attached to the substrate.
It is desirable to heat-treat only a portion of these workpieces when forming thread-cutting fasteners rather than an entire threaded area and to prevent the remainder from becoming brittle. A required property for these fasteners is that the remaining portion that is not treated remain ductile so that they do not break when fastened down. That is, although heat-treated and hardened sections become good thread-cutters, the hardened sections do not make good fasteners because they become too brittle. Untreated ductile cores of workpieces have improved strength and torsion characteristics as compared to hardened cores, thereby lessening the likelihood of failure when torquing the fastener, removing the fastener from the substrate, or while the fastener is torqued down.
Another property desired in each of the workpieces is that they be hardened in a specific region to a desired thread root depth, and have a consistent hardness from piece to piece. To the extent that any of the above desired properties are found lacking in a particular workpiece, or from piece to piece, the rate of defects increases and the cost associated with quality control increases.
Induction hardening of workpieces is well-known in the art. One known method to harden threaded fasteners, such as screws and bolts, includes the use of a helical induction coil having the threaded fastener closely coupled thereto. The threaded fastener is axially inserted within the induction coil to a desired depth for heat-treating the workpiece. These devices are not well suited for high production. For workpieces having heads such as bolts or screws, after heating by the closely coupled helical coil, quenching of such workpieces can also be problematic. These workpieces cannot be readily passed through the helical coil to a quench station, but must either be quenched in position or manually removed from the helical coil and carried to a quench tank for cooling.
The process of manually removing workpieces from the helical coil and dropping them into a quench tank is inefficient and slow. Manually removing workpieces after heat-treatment, however, is widely used since the time lapse from discontinued heating of the workpiece to quenching is critical to controlling the uniformity, depth, and hardness levels of the heat-treatment. Also critical is assuring accurate positioning of the workpieces during heat-treating. Minor variations in the alignment and positioning of the workpieces within the induction coil can result in defects, such as brittle threads or inadequate hardening.
Another known method to harden workpieces is the use of a channel-like inductor coil in the form of one or more elongated turns between two parallel sides having workpieces laterally traveling therethrough. This method provides an increased rate at which workpieces can be heat-treated, but causes a reduction in the uniformity of heat-treatment. Such methods typically use gravity to translate the workpieces laterally through the induction coil and to a quenching step after leaving the induction coil. Alternatively, the workpieces can be dropped from the induction coil into a quench tank.
Using gravity to convey the workpieces can cause other problems as well. For example, momentary stoppages in flow, followed by agitated movements of the workpieces, can cause defects as the workpieces pass through the induction coil. When the workpieces travel at different speeds, the time that the workpieces are heat-treated varies resulting in uneven heat-treatment. Additionally, workpieces may overlap one another misaligning the workpieces as they pass through the induction coil. That is, if the head of one fastener rides up on the head of another, the first fastener may be too high or cocked, thereby preventing adequate and/or even heat distribution.
Another known method provides a conveyor system for heat-treating a zone of articles. A feeder bowl supplies workpieces which are translated through an induction heating coil by a first conveyor and then quenched by a liquid spray which directs a cooling liquid onto the heated ends of the workpieces. Alternatively, the workpieces may have their ends dipped into a cool liquid reservoir or be subjected to a blast of cold air or any other type of cooling gas. After the workpieces are heated and quenched, a second conveyor comprising a continuously rotatable table removes the workpieces from the first conveyor at a rate determined by the rotational speed of the rotatable table. The rotatable table determines the rate at which articles pass through the induction coils according to the rate at which articles are removed from the first conveyor. Further methods eliminate the pick-up member by driving the articles through the induction heating coil using the first conveyor. Similar problems exist using these methods as with the other methods discussed. Since the articles are fed by gravity, they can overlap and/or travel at different rates of speed while passing through the induction coil which can result in an increased rate of defects. Additionally, it is difficult to efficiently identify and separate adequately heat-treated workpieces from inadequately heat-treated workpieces. The inability to quickly identify inadequately heat-treated workpieces limits the production rate at which workpieces may be manufactured, and results in the implementation of expensive manual quality control testing and procedures for separating adequately heat-treated workpieces from inadequately heat-treated workpieces.
It would therefore be desirable to have a device and method for heat-treating workpieces, such as threaded fasteners, in which the workpieces are separated and the rate at which they are treated is governed to provide consistent or even heat-treatment.
The present invention provides a system and method for hardening and separating workpieces to produce uniformly heat-treated workpieces solving the aforementioned problems.
A rotary conveyor is provided to transport workpieces in succession through an induction heater. The workpieces are aligned so that at least a portion of each workpiece is heat-treated upon passing through the heater, such as an induction heating coil. The rotary conveyor includes individual retention members to separate the workpieces as they travel through the heater. A sensor examines the heat-treated portion of each of the workpieces to determine whether the workpieces are adequately heat-treated. Based on the determination made by a control connected to the sensor, an ejector separates the workpieces into adequately heat-treated or non-defective workpieces, from inadequately heat-treated or defective workpieces. The workpieces are then quenched in a quenching mechanism.
In accordance with one aspect of the present invention, a heat-treating apparatus is disclosed comprising a rotary conveyor having a discrete number of workpiece retainers or slots for carrying the workpieces as they travel around the rotary conveyor from a reception station to a discharge station. The apparatus also comprises a heater, such as an induction heating coil, positioned under the rotary conveyor to heat a workpiece positioned in the workpiece retainer of the rotary conveyor while the workpiece travels from the reception station to the discharge station.
In accordance with another aspect of the present invention, a system to inductively heat and quench-harden at least a portion of a workpiece includes a circular conveyor system and an arc-shaped induction heater positioned adjacent to the circular conveyor system. The arc-shaped induction heater heat-treats at least a portion of each workpiece traveling therethrough on the circular conveyor system. The system also includes a quencher located to receive and cool each workpiece discharged from the circular conveyor system.
In another aspect of the present invention, a control for hardening workpieces is disclosed. The control causes a rotary conveyor to position workpieces in aligned succession and convey the workpieces through an arc-shaped heater. The arc-shaped heater heat-treats the workpieces, which can then be cooled in a quench tank or by another cooling mechanism.
In a further aspect of the present invention, a heat-treating device is provided having a means for rotationally conveying workpieces or threaded fasteners in an aligned succession, and a means for heat-treating the workpieces while being conveyed in the means for rotationally conveying workpieces. If desired, the heat-treating device can also include a means for separating non-defective or adequately heat-treated workpieces from defective or inadequately heat-treated workpieces. The devices preferably also include a means for cooling the workpieces.
In yet another aspect of the present invention, a method of heat-treating a workpiece includes positioning workpieces on a rotatable conveyor system and rotating the rotatable conveyor system such that each workpiece travels through a heater. The heater is designed to apply a heat-treatment to at least a portion of each workpiece while moving through the rotatable conveyor system. The method can also include the step of quenching the workpieces in a quenching mechanism to cool the workpieces.
Various other features, objects and advantages of the present invention will be made apparent from the following detailed description and the drawings.